Cyclopeptides are polypeptides in which the terminal amine and carboxyl groups form an internal peptide bond. Several cyclopeptides are known for their advantageous medicinal properties. An excellent example of this is the class of echinocandins which are potent antifungals. Echinocandins inhibit the synthesis of glucan in the cell wall through noncompetitive inhibition of the enzyme 1,3-β-glucan synthase. Echinocandins are used in candidiasis and aspergillosis, they are fungicidal against some yeasts (most species of Candida), fungistatic against some molds and modestly or minimally active against dimorphic fungi (Blastomyces and Histoplasma). Also they have activity against the spores of the fungus Pneumocystis carinii. 
Echinocandins can be naturally occurring compounds but may also be obtained by total synthesis or by synthetic or genetic modification of naturally occurring or naturally produced precursors; the latter class is referred to as semi synthetic echinocandins. One of the first echinocandins of the pneumocandin type, echinocandin B, could not be used clinically due to risk of high degree of hemolysis. However, preparation and screening of semi synthetic analogs of the echinocandins resulted in to cilofungin, the first echinocandin analog to enter clinical trials. Later semi synthetic pneumocandin analogs of echinocandins were found to have similar or improved antifungal activity with lower toxicity. The first approved of these newer echinocandins was caspofungin, and later micafungin and anidulafungin were also approved. Anidulafungin, caspofungin and micafungin are all semi synthetic echinocandins derivable from naturally occurring echinocandins such as echinocandin B, pneumocandin A0, or pneumocandin B0. One drawback of the compounds currently available to the market is low oral bioavailability, and hence they must be administered intravenously. Nevertheless, echinocandins have now become one of the first line treatments for Candida, but also for treating fungal infections caused by Aspergillus, Blastomyces, Coccidioides and Histoplasma. 
Although nature can provide a substantive part of the complex chemical structure of semi synthetic cyclopeptides, and in many cases having all chiral centers in the required configuration, a major disadvantage nevertheless is that during fermentation often side products are formed that carry through the process and eventually end up as impurities. Only in few cases can fermentation processes be tuned in such a way as to prevent formation of impurities. Particularly when these impurities are structurally closely related to the main product, their removal is usually tedious and often requires unprecedented purification approaches as the main products in question are chemically unstable and/or prone to racemization.

More specifically, the preparation of caspofungin ((1), R═H) from fermentatively obtained pneumocandin B0 is a process wherein control of impurities is an important issue and indeed control of impurities has been the subject of several earlier studies such as, for example, US 2009/0291996 and US 2009/0324635 and references cited therein. Further improvement of impurity control in caspofungin production is one of the foundations of the invention as herein described. Upon preparation of caspofungin registration batches it was observed in certain HPLC traces that a shoulder is present on the descending side of the main caspofungin peak; this shoulder has hitherto not been described in literature. Although the cause of this phenomenon, i.e. the formation of an unwanted by-product, is at present unknown, the problem underlying the present invention is the determination of the chemical structure of the compound in question and the design of analytical methodology to establish presence or absence of this impurity in caspofungin batches.
In addition, another objective is to provide new pharmaceutically active compounds with the goal to further improve the spectrum of antifungal drugs available for treatment of fungal infections. The latter is of relevance in view of the problem of continuously developing resistance against existing drugs.